Biomedical Engineering Reference
In-Depth Information
Fig. 2.
Effect of cAMP and NRG-1-
ȕ
treatment versus control on
I
f
current density (
a
-
d
).
Original patch-clamp recordings in the whole cell configuration (
a
-
c
). Compared to control (a),
cAMP (
b
) and NRG-1-
ȕ
(
c
) significantly increased the
I
f
current density.
In addition to changes of Cx-40 mRNA levels we demonstrated a significant
increase of
I
f
current density by neuregulin and cAMP. The pacemaker current
I
f
plays
a central role for spontaneous diastolic depolarization of sinus node cells [26]. While
small
I
f
currents may also be detected in working myocardium [3, 13],
I
f
current size
is markedly larger in pacemaker cells [6]. Acute stimulation of
I
f
by cAMP is well
known to shift the activation curve to more positive potentials and, thus, to increase
current amplitude at half maximal activation [8, 12]. cAMP binds to the cytoplasmatic
site of HCN channels which permits the channels to open more rapidly and
completely after repolarization of the action potential with the result of accelerating
rhythmogenesis [28]. However, maximal current size remains unaffected by acute
sympathetic stimulation [8, 12]. In the present experiments we omitted any acute
effects of both cAMP and neuregulin. In contrast to typical acute modulation of
I
f
we
observed a significant increase of maximal current density at full activation potentials
indicating elevated channel expression. Evaluation of expression differences of HCN
isoforms will be done as a next step to verify the mechanism by which cAMP and
neuregulin-1 influence
I
f
in embryonic cardiomyocytes.
Our present observations give further insight into the differentiation of the CCS.
Moreover, our approach constitutes an elegant way to direct differentiation towards
pacemaker-like cells out of a heterogeneous cell population. Generally, embryonic
cardiomyocytes might not prove optimal candidates for cell transplantation due to low
availability, their allogenic nature and possible ethical considerations. However,
embryonic and fetal cardiomyocytes are so far the best characterized cells showing
coupling with the recipient myocardium even on the ultrastructural level [27].
Therefore, they can serve as a good model for the identification of factors enabling
generation of pacemaker cells from early stage atrial or ventricular cardiomyocytes.
